It would potentially mean WhatsApp and Signal disappearing from the App Stores in the UK, and UK based accounts being shut down. Whatsapp could concievably just switch off encryption for the UK users or make a UK only version with no contact to the rest of the network (but that would be a major back down), but Signal would never be able to accept that and would likely exit. However Meta may still decide it's better to exit WhatsApp completely than stay if the regulatory burden and risks are too high.

It is possible however that these bills won't make it through parliament either at all or in there current form. We're about 1 year away from a general election (the latest the election can be held is Jan 2025 but they desperately want to avoid a christmas election campaign, so late autum 2024 seems likely) - that means campaigning effectively starts soon and controversial legislation like this can become quickly toxic to MPs desperate to hold their seats. It really depends on what the general reaction to this is - at the moment most people don't seem to care but the social media companies do have the power to drum up public opposition should they wish to fight this.